Dr. Terry L. Goforth
Associate Professor of Physics, SWOSU
July 2004

On July 31, 2004, most of the world will witness an event that literally happens only once in a blue moon. The event? A blue moon, of course! Will the moon actually appear blue? Probably not. So what do we mean by a blue moon?

The "modern" definition of a blue moon is "the second full moon occurring in a calendar month. We have already witnessed one full moon in July, back on July 2. The next full moon will occur at 1:05 p.m. CDT on July 31. For most of the world, this will be the second full moon of July, and therefore, a blue moon. (Although the full moon–defined as the moment the moon is at its fullest–occurs at the same instant for everyone, the local time of the full moon depends on your time zone. For portions of eastern Asia, Japan, Australia, and on east to the international date line, the date will already be August 1 when the moon reaches maximum fill phase. Therefore, these areas will actually experience two full moons in August. For them, the blue moon will occur on August 30–remember, it's the second full moon of the month that is called the blue moon.)

How often do blue moons occur?
The average lunar cycle (time from one full moon to the next) is about 29.53 days. The word month is actually derived from the same root as moon, but our modern calendar has removed the link between a month and the lunar cycle. Our months are now 30 or 31 days (except of course for February), which means the month is just a little longer than a lunar cycle. If the first full moon of a month occurs on the first or second day, the next full moon will occur on the 30th or 31st of the month. Of course, two full moons are more likely to occur in 31-day months and can never occur in February, leap year or not. One year (365.25 days on the average) contains 12 months, but is about 12.37 lunar cycles. So we accumulate an "extra lunar cycle" about once every 32 or 33 months. That is to say, in 33 calendar months we will witness 34 full moons–so at least one month in that time period must have two full moons. It is actually possible to have two months with two full moons in a 33 month period if one of the Februarys in that time has no full moon. (This is possible because the month of February is shorter than the lunar cycle.) So a blue moon occurs about once, occasionally twice, every 33 months.

Can the moon actually turn blue?
No. The moon is a dead chunk of rock with a dark gray color. However, it can appear blue to Earth-bound observers under some very rare atmospheric conditions. Light of different colors will interact with particles suspended in the Earth's atmosphere. This is what gives us blue skies. If a lot of dust or pollution is stirred up, blue light "scatters," or bounces around more easily than red light. With the blue light removed and mostly red remaining, we see red sunrises and sunsets. Slightly larger particles will have the opposite effect. Red light will scatter leaving behind more of the blue. Getting a large collection of these larger particles is not common. It generally only happens following major ash-laden volcanic eruptions or sometimes during particularly virulent forest fires. Both the sun and moon were reported to appear blue worldwide for some time following the eruption of Krakatoa in 1883. Residents of eastern Canada and the U.S. and in England also observed blue suns and moons in 1951 caused by smoke particles released during massive forest fires in Alberta. The earliest recorded references to the expression "blue moon" date back to the 16th Century and were clearly used to describe events that were preposterous or would never occur. By the mid-19th Century the expression "once in a blue moon" was in use and meant an event that was rare. It is almost certain that this meaning was derived from the occasional observation of a blue-colored moon caused by events such as those described above.

Why is the second full moon of a month called "blue?"
The modern definition of a blue moon actually only dates back to the 1940's. In a question-and- answer column for Sky and Telescope in July 1943, Laurence J. Lafleur of Antiock College quoted the August calendar page of the Maine Farmer's Almanac where it says "However, occasionally the moon comes full thirteen times in a year." The almanac reported a blue moon for the month of August that year. Three years later in the March 1946 edition of Sky and Telescope, James Hugh Pruett, an amateur astronomy and frequent contributor, referred to Lafleur's article, and added to it the comment that "Seven times in 19 years there were–and still are–13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon." Pruett inferred the definition that a blue moon is the second full moon in a calendar month without referring to the original source, the Maine Farmer's Almanac–which listed the blue moon occurring on August 21, 1937, and was clearly not the second full moon of that month! However, Pruett's definition was picked up by Deborah Byrd in her January 31, 1980, broadcast of Star Date on NPR. From here, it was included in The Kids World Almanac of Records and Facts (1984 by Margo McLoone-Basta and Alice Siegel), and then included in the Genus II edition of Trivial Pursuit in 1986. At this point there was no turning back. This modern definition is "out there" and is now the accepted definition of a blue moon.

So what was the definition of a blue moon before then?
The meaning of a blue moon as intended by the Maine Farmer's Almanac is not known with certainty, but Donald W. Olson, Richard Tresch Fienberg, and Roger W. Sinnott tried to track it down in an article in Sky and Telescope in May 1999. (This article was in response to a March 1999, Sky and Telescope article by Philip Hiscock which had uncovered Pruett's unintentional introduction of a new definition and its propagation as described above.) It seems that the Almanac did refer to the "extra" full moon in a year to be a blue moon, but which one is extra? First, the Almanac actually didn't use a calendar year. Instead, to identify a blue moon it considered a tropical year which begins on the winter solstice–the first day of winter in the northern hemisphere. Normally there will be three full moons per season. Following Pope Gregory XIII's calendar reform of 1582, the Christian ecclesiastical calendar identifies these full moon with names such as the First Full moon after Yule (the first full moon after the winter solstice), the Lenten Full Moon (full moon during lent, always the last full moon of winter), the Paschal Full Moon (the first full moon of spring, always occurring less than a week before Easter), and so on. If a season has four full moons, it throws off the dates of religious holidays and feasts. So, if a season has four full moons, the third full moon of the season is referred to as a blue moon, allowing the first, second, and last full moon of each season to retain its identity. This is critical to determining the time of Easter, which is then used to determine the dates of other moveable Christian holidays such as Lent and Ascension. Based on this definition of a blue moon, there are still seven blue moons every 19 years, they just occur at different times than those of the more modern definition. The next time we'll have four full moons in a season is summer of 2005. The third full moon of that season will be on August 19, 2005, at 12:53 p.m. CDT. This is the next blue moon based on the (assumed) definition used by the Maine Farmer's Almanac.

So when is the next blue moon?
Choose your definition. Or use them both–that way we get a blue moon twice as often (making it a less rare event). Either way, you'll get a chance to observe the second full moon of July this week. Feel free to call it blue.

   * Sky and Telescope, March 1999, "Once in a Blue Moon" by Philip Hiscock, p 52 (online at
   * Sky and Telescope, May 1999, "What's a Blue Moon" by Donald W. Olson, Richard Tresch Fienberg, and Roger W. Sinnott, p 36 (online at

Extra Material
There are many collection of names given to the full moons. Most define a name for the full moon of each calendar month. From a web site ( by Richard Minutillo of The Fine-Arts Bluesband and Poetry Press, I found this list based on the seasons.

Season Name
Winter 1st Moon after Yule
  2nd Wolf Moon
  Last Lenten Moon
Spring 1st Paschal Moon (Egg Moon, Easter Moon)
  2nd Milk Moon
  Last Flower Moon
Summer 1st Hay Moon
  2nd Grain Moon
  Last Fruit Moon
Autumn 1st Harvest Moon
  2nd Hunter's Moon
  Last Moon before Yule
Insert Blue Moons in any season as required, after the 2nd moon and before the last.
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Last update:  July 20, 2004